Quick way to figure out the rc.conf directive: %/usr/local/etc/rc.d/mongrel_cluster rcvar

Anyway, it sounds like you've already got it, since you can start the service manually. If it's not starting at boot time, it's possible there is another service it depends on that is not yet up. I'm not familiar with mongrel -- what does it rely on to start properly? You may need to fix the REQUIRE line.

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P.S. Check the tail of of this command to look for clues about why it may not be starting: %dmesg -a | less